An access cannula assembly, as implied by its name, is a medical/surgical tool used to form a portal into tissue within the patient. It is typically desirable form this portal so a procedure can be formed on the tissue adjacent the distal end of the portal. For example, one type of procedure for which it is desirable to form such a portal is biopsy procedure. In a biopsy procedure, once the portal is formed, a biopsy instrument is inserted in the portal. This biopsy instrument is used to retrieve a sample of the tissue. The retrieved tissue is then studied to determine the pathology of the tissue in the patient. Another type of procedure in which it is desirable to form a portal into a patient is a vertebroplasty procedure. A vertbroplasty procedure is a procedure that is performed on a fractured vertebral body. In this procedure, the access cannula is used to define a portal or passageway into a fractured vertebral body. Once the portal is formed, a stabilizing cement is introduces through the portal into the vertebral body. The cement, upon hardening, holds the fractured sections of the vertebra together. The holding together of these portions of the vertebra stabilizes the fracture to reduce the pain resulting from the fracture.
Some access cannula assemblies are designed specifically to create an opening into hard tissue, bone. This type of assembly typically includes two basic components: the actual cannula; and a stylet. The cannula is the tube like structure. The lumen or bore that extends through the cannula functions as the portal to the tissue for which access is required to perform the intended medical or surgical procedure. The stylet is often a solid rod. During the insertion of the access cannula assembly into the bone, the stylet is removably disposed in the cannula bore. The stylet is further formed to have a pointed tip. More particularly, the tip has a geometry that facilitates the driving of the stylet through the bone. The components forming an access cannula assembly are further formed so that when the stylet is seated in the cannula, the tip typically extends distally forward of the end of the cannula. Both the access cannula and tip are formed with handles. The handles facilitate the grasping of these components.
An access cannula assembly used to form a portal into bone into is typically driven into the bone and overlying soft tissue with an axial force that is applied against at least one of the cannula handle or the stylet handle. During this process the tip of the stylet functions as a drill bit or wedge that separates the tissue so the cannula can advance. The shaft of the stylet provides mechanical strength to the assembly so the cannula does not collapse under exposure to an axial load.
This force used to insert the assembly may be the simple manual force applied with the palm of the hand. In a variation on this technique, the assembly is both pushed against the bone and rotated. The rotation causes the stylet tip to drill into the tissue. Alternatively, a practitioner may use a mallet like device to drive the assembly into the bone.
Regardless of the method used to drive the access cannula into the bone, the stylet, more typically, the tip, is subjected to a greater resistive force than the resistive force to which the cannula is exposed. In response to the uneven application of these forces, the stylet tip can start to retract proximally such that the tip retracts into the cannula bore. If this event occurs, to continue the insertion process, there is a need to momentarily provide an additional quantum of force to the stylet and only the stylet in order to drive the tip forward of the cannula. Having to add this sub-step to the insertion process can increase the overall time it takes to perform the insertion process. Moreover, having to interrupt the insertion process to have to so reset the position of the stylet relative to the access cannula can interrupt the flow of the insertion process.
To minimize, if not eliminate, the instances of having to so reset the position of the stylet relative to the cannula, an access cannula assembly typically includes a lock assembly that releasably holds the stylet to the cannula. A lock assembly often consists of providing either the stylet handle or the cannula handle with one or more tabs. The other of the cannula handle or stylet handle with one or more voids for removably receiving the tabs. The components are designed so there is an interference fit between the tabs and void-defining features of the complementary handle. During the process of inserting the access cannula into the bone, the interference fit hold the tabs in the complementary voids. The tab-in-void seating is designed to reduce the likelihood that, as force is applied to either the cannula or the stylet, these components move independently from each other. Once the access cannula is at the desired location, the lock assembly is released. This allows the stylet to be withdrawn from the access cannula. The cannula bore is then clear to receive the next instrument or compound the practitioner wants to introduce into the site to which the cannula is directed.
Lock assemblies integral with many access cannula assemblies work reasonably well under many circumstances. However, in some situations, known cannula assembly lock assemblies may have a less than desirable performance. For example, one known cannula assembly has tabs that project radially outwardly from the handle integral with the stylet. These tabs extend into slots formed in the cannula handle. This type of lock can have a less than desired performance when the access cannula assembly with which the assembly is integral is twisted into bone. During this type of insertion process, as a result of the stylet being subjected to more resistive torque than the interference fit holding the tabs, the cannula handle may rotate free of the stylet lock tabs. Further, once the access cannula is positioned at the target location, the stylet tip may be firmly embedded in the adjacent bone. To free the stylet it is necessary to apply a twisting force, a torque, to the stylet. This force must be sufficient to overcome the both the resistance of the interference fit and the resistance of the bone. This type of access cannula presents a stylet handle that has a relative small area to the practitioner. This small sized handle can make it difficult to exert the necessary removal torque.
In a variation of this type of access cannula, the smaller diameter stylet handle is simply threaded to the larger sized access cannula. Again, during a twisting insertion process, due to the resistance of the bone overcoming the interference of the interference fit, the lock can disengage. This type of access cannula also has the disadvantage of the above described tab-in-void space lock. The relatively small size of the stylet handle can make it difficult to correctly apply the force required to disengage the lock and withdraw the stylet.
Another known cannula assembly includes a stylet with a handle that extends over the cannula handle. Lock tabs integral with the stylet handle project downwardly from this handle. These lock tabs extend into underlying slots formed in the cannula handle. During the twisting in of a cannula assembly with type of lock, care must be taken to rotate both handles simultaneously. If the cannula and stylet are not rotated simultaneously, the rotation of the stylet relative to the cannula can result in the unintended disengagement of the lock.
Access cannula assembles with locks designed to substantially eliminate the likelihood of unintended disengagement are available. The lock integral with this type of access cannula assembly often include a latch component that is flexible. A disadvantage of the known version of this type of assembly is that, to withdraw the stylet, it is necessary to first retract the stylet proximally. During a procedure debris, such as bone dust, can become entrained in the narrow space between the stylet and the inner wall of the cannula. These debris can impede the proximal withdrawal of the stylet.
Further, sometimes the tip of the stylet may be embedded in bone. One relatively easy means to free the tip from the bone is to rotate the stylet to break up the bone around the tip. However, since to withdraw the stylet of the above-described assembly, it is necessary to first displace the stylet longitudinally, it may not be possible to so rotate the stylet. This can make it difficult to free the tip from the bone.